As an example of the conventional impact tools, Japanese Patent Application Publication No. JP H11-58262 A discloses an impact tool that effectively suppresses shock when striking is started from a state in which no-load striking is prevented. In this impact tool, an impact element is accommodated in front of the piston in a cylinder, and the air chamber is interposed between the impact element and the piston. The impact element is movable in the longitudinal direction through an air chamber. Auxiliary holes, front air holes, and rear air holes are provided at the position of the air chamber in the cylinder so as to be shifted in position from each other in the axial direction of the cylinder.
When this conventional impact tool starts striking, these holes are sequentially closed, and the impact tool is gradually switched to a sealed state (a full-load striking state). Thus, the impact element is not rapidly retracted rearward by the piston reciprocating in the cylinder.
Further, in order to implement a proper pressure fluctuation in the air chamber during reciprocation of the piston, this impact tool has a single vent hole near the top of the upper half of the peripheral surface of the cylinder to allow the air chamber to communicate with the outside of the cylinder. While the piston reciprocates, a part of the air in the air chamber is discharged through the vent hole, or the air outside the cylinder is introduced into the air chamber through the vent hole.
However, taking the air into or out of the air chamber through the single vent hole may generate a pressure difference in the air chamber because the pressure in the air chamber decreases partially. The piston and the impact element are subjected to uneven pressures due to the pressure difference. If the piston and the impact element are tilted by such different pressures, and reciprocate while being pressed against the inner surface of the cylinder, heat may be generated by the friction of the piston and the impact element with the cylinder.
Such friction of the piston and the impact element with the cylinder may reduce the operating speed of the piston and the impact element, thereby reducing operation efficiency of the impact tool.